256 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6OO 550 .•500 1909 nm 450 400 'D_ 350 c• 300 250 •• 2OO 1 obsolufe humidify, g/m3 Figure 5. Correlation of bound water (1909 nm) and absolute humidity. 10 original and MSC-corrected spectra (Figure 10). The relative change in scattering among the three formulations is independent of the wavelength used, and C A B, where B, the penetration enhancer, shows the least decrease in scattering compared to the untreated site. The two formulations containing the plasticizer, A and C, cause a greater decrease in scattering and are comparable to each other. DISCUSSION The validity of using NIR reflectance data for assessing skin condition depends upon both the depth from which the measured radiation is coming and the degree of change in this depth upon product application. Ideally, in evaluating moisturizing products, the depth of penetration of near-infrared radiation should be limited to the stratum corneum. Increased depth of penetration could result in a higher apparent concentration of water, especially that which is termed here as bulk, due to the greater degree of hydration of deeper tissue. The depth of penetration of radiation into the skin depends on several factors. When NIR radiation impinges on skin, it undergoes regular (specular or mirror-like) reflec- tance, absorption, internal scattering, and diffuse reflectance. The degree to which each occurs depends on the structure, the refractive index, the absorptivity, and the scatter- ing coefficient. Figure ! ! illustrates some possible pathways for light reflected off skin. Anderson et al. (! 1) suggest that at the 1950 nm combination band of water, specular reflectance from

MOISTURE MEASUREMENT BY NIR SPECTROSCOPY 257 600 c 550 500 450 400 350 300 m m m 1927 nm m m •/'m • m m m m .••m m m 1 ab$olufe humidify, g/m3 Figure 6. Correlation of bound water (1927 nm) and absolute humidity. 10 600- 400 t 8001 x 6001 4001 200! absolute humidity, g/m3 Figure 7. Correlation ooe bulk water (1890 nm) and absolute humidity. the epidermis dominates, with only 5%-7% diffuse reflectance. Light that does not penetrate into the SC can undergo single scattering or multiple internal scattering. The stratified nature of the stratum corneum, with its layers of flattened cells, provides several opportunities for internal scattering. Some light may also be completely absorbed